The present invention relates to a method for manufacturing a gas sensing element of a sensor. The gas sensor is generally installed in an exhaust gas passage of an internal combustion engine for combustion control or emission control of the internal combustion engine.
A gas sensing element is necessary to control the combustion control of an internal combustion engine. The gas sensing element has a cup-shaped solid electrolytic body having a reference gas chamber formed therein. An inside electrode is provided on an inner surface of the solid electrolytic body. An outside electrode is provided on an outer surface of the solid electrolytic body.
However, according to a conventional gas sensing element, there is the possibility that the outside electrode may separate from the solid electrolytic body.
In view of the foregoing, the present invention has an object to provide a method for manufacturing a gas sensing element having an excellent bonding strength between an outside electrode and a solid electrolytic body.
In order to accomplish the above and other related objects, the present invention provides a method for manufacturing a gas sensing element which has a cup-shaped solid electrolytic body having a reference gas chamber formed therein, an inside electrode provided on an inner surface of the solid electrolytic body, and an outside electrode provided on an outer surface of the solid electrolytic body. The manufacturing method of this invention comprises a step of forming a non-sintered element body having a predetermined shape from powdery raw material of the solid electrolytic body, a step of temporarily sintering the non-sintered element body to obtain a partially-sintered element body as a semi-finished product of the solid electrolytic body, a step of dipping an outer surface of the partially-sintered element body into a slurry containing surface-roughing powder including large and small grains which are mutually differentiated in grain size, and a step of completely sintering the partially-sintered element body with a rough slurry film coated thereon into the solid electrolytic body.
According to this invention, the outer surface of the partially-sintered element body is dipped into the slurry containing the surface-roughing powder including large and small grains. The slurry film with the mixed large and small grains is formed on the outer surface of the partially-sintered element body. Then, the complete sintering treatment is performed. Accordingly, the outer surface of the completely sintered solid electrolytic body is finished into a rough surface whose roughness depends on the grain size and the mixing ratio of the large and small grains.
The outside electrode is fixed to the outer surface of the sintered solid electrolytic body, after the outer surface is finished into the rough surface. The rough surface brings anchor effect. In other words, the rough surface assures an excellent bonding strength required for firmly fixing other member thereon. Thus, according to the manufacturing method of the present invention, the outside electrode can be firmly fixed on the outer surface of the solid electrolytic body due to the anchor effect brought by the coated rough surface. In other words, this invention provides an excellent method for manufacturing a gas sensor which is capable of effectively preventing the outside electrode from peeling off the solid electrolytic body and is also capable of assuring excellent durability.
According to the manufacturing method of the present invention, the surface roughness of the solid electrolytic body can be easily changed or adjusted by adequately selecting the grain size of the large and small grains and their contents relative to the slurry. Thus, the manufacturing method of this invention is easily realized and brings the effect of cost reduction.
As understood from the foregoing, according to the present invention, it becomes possible to obtain a manufacturing method of a gas sensor which assures an excellent bonding force for the outside electrode bonded on the outer surface of the solid electrolytic body.
According to the present invention, for the purpose of preventing the outside electrode from being directly exposed to the measured gas, it is preferable to provide a trap layer and/or a protective layer so as to cover the outside electrode. The trap layer traps poisonous or harmful substances contained in the measured gas.
Furthermore, it is also preferable to provide a diffusion resistive layer so as to cover the outside electrode. The diffusion resistive layer controls the time required for the measured gas to reach the outside electrode. The diffusion resistive layer further controls the amount of the measured gas reaching the outside electrode.
In this case, according to the manufacturing method of the present invention, the outer surface of the solid electrolytic body is finished into a rough surface. The rough surface brings a strong bonding force required for firmly fixing the outside electrode on the outer surface of the solid electrolytic body. The rough surface also brings a sufficient bonding force required for firmly fixing the additional layers on the outside electrode. The additional layers provided on the outside electrode include the trap layer, the protective layer, and the diffusion resistive layer.
Accordingly, the present invention makes it possible to prevent the outside electrode from peeling off the outer surface of the solid electrolytic body. Furthermore, the present invention makes it possible to prevent each additional layer from peeling off the outside electrode or the outer surface of the solid electrolytic body.
Furthermore, the portion of the solid electrolytic body to be dipped into the slurry containing the surface roughing powder can be limited to a specific region where the outside electrode is provided.
It is preferable that a coating area of the rough surface is sufficiently wide to entirely cover the region where the outside electrode is provided. However, the effect of the present invention can be obtained even when the coating area of the rough surface is somewhat smaller than the entire area of the outside electrode.
Furthermore, when the trap layer, the protective layer, and the diffusion resistive layer are provided on the outside electrode or on the outer surface of the solid electrolytic body, it is preferable to dip the corresponding portion of the solid electrolytic body into the slurry containing the surface roughing powder.
With this arrangement, it becomes possible to assure a strong bonding force required for firmly fixing these additional layers together with the outside electrode on the outer surface of the solid electrolytic body.
Furthermore, the gas sensing element is generally equipped with electric leads and terminals connected to the inside and outside electrodes for outputting a sensing signal from the electrodes or applying a voltage to the electrodes. Thus, some of the leads and terminals are provided on the outer surface of the solid electrolytic body.
In this case, to increase the bonding strength for fixing the leads and terminals on the outer surface of the solid electrolytic body, it is preferable to dip the portion of the solid electrolytic body corresponding to the leads and the terminals into the slurry containing the surface roughing powder.
Furthermore, it is preferable that the material of the surface roughing powder is identical with that of the solid electrolytic body. It is also preferable that the surface roughing powder can be integrated with the solid electrolytic body through the sintering treatment.
This is effective to prevent the slurry film containing the surface roughing powder from peeling off the solid electrolytic body.
Furthermore, a binder is generally added with the grains to form the slurry containing the surface roughing powder. A preferable binder is PVA (polyvinyl alcohol).
According to the manufacturing method of this invention, it is preferable that a coating density of the slurry during the step of dipping the outer surface of the partially-sintered element body is in a range of 0.05 mg/mm2 to 0.30 mg/mm2 in terms of the amount of the surface-roughing powder contained in the slurry.
Setting the coating density to a value in the above-described preferable range is effective to prevent the outside electrode from peeling off the outer surface of the solid electrolytic body. Furthermore, it is effective to enhance the bonding strength of the protective layer.
If the coating density is less than 0.05 mg/mm2, it will be difficult to obtain a sufficient bonding force for fixing the outside electrode and the protective layer on the solid electrolytic body. On the other hand, if the coating density is larger than 0.30 mg/mm2, the strength of the coated rough surface will be worsened.
Furthermore, according to the manufacturing method of this invention, it is preferable that the grain size of the large grains is in a range from 5 xcexcm to 50 xcexcm.
In this case, it becomes possible to form a preferable outside electrode. It becomes possible to assure a strong bonding force required for fixing the outside electrode and the protective layer on the solid electrolytic body.
If the grain size of the large grains is less than 5 xcexcm, it will be difficult to form the rough surface having a sufficient surface roughness required for preventing the outside electrode from peeling off the solid electrolytic body. On the other hand, if the grain size of the large grains is larger than 50 xcexcm, it will be difficult to form an appropriate outside electrode.
Furthermore, according to the manufacturing method of this invention, it is preferable that the grain size of the small grains is not larger than 1 xcexcm.
This is effective to assure an appropriate holding force for holding the large grains. If the grain size of the small grains is larger than 1 xcexcm, it will be difficult to obtain a sufficient holding force for holding the large grains.
Furthermore, according to the manufacturing method of this invention, it is preferable that the grain size of the small grains is in a range from 0.1 xcexcm to 1 xcexcm.
This is effective to assure an appropriate holding force for holding the large grains. If the grain size of the small grains is less than 0.1 xcexcm, it will encounter the problem in handling the powder, such as flying off of micro powder. On the other hand, if the grain size of the small grains is larger than 1 xcexcm, it will be difficult to obtain a sufficient holding force for holding the large grains.
Furthermore, according to the manufacturing method of this invention, it is preferable that the entire content of the large grains is in a range from 5 weight % to 20 weight % when the slurry is 100 weight %.
This is effective to obtain an adequate bonding force required for fixing the outside electrode and the protective layer on the solid electrolytic body.
If the entire content of the large grains is less than 5 weight %, it will be difficult to obtain a sufficient bonding force required for fixing the outside electrode and the protective layer on the solid electrolytic body. On the other hand, if the entire content of the large grains is larger than 20 weight %, the large grains may fall off the slurry film.
Furthermore, according to the manufacturing method of this invention, it is preferable that the entire content of the small grains is in a range from 10 weight % to 20 weight % when the slurry is 100 weight %.
This is effective to assure an adequate holding force required for holding the large grains. If the entire content of the small grains is less than 10 weight %, the large grains may fall off the slurry film. On the other hand, if the entire content of the large grains is larger than 20 weight %, it will be difficult to assure a sufficient bonding force required for fixing the outside electrode and the protective layer on the solid electrolytic body.
According to the manufacturing method of the present invention, it is preferable that the step of dipping the outer surface of the partially-sintered element body into the slurry containing the surface-roughing powder is performed in such a manner that a slurry film is formed on the outer surface of the partially-sintered element body, the large grains protrude from a surface level of the slurry film and are spaced from each other, and the following relationship is satisfied,
0.25dxe2x89xa6txe2x89xa60.75d 
wherein xe2x80x98dxe2x80x99 represents a grain diameter of the large grains and xe2x80x98txe2x80x99 represents a thickness of the slurry film.
Dipping the partially-sintered element body into the slurry results in formation of the slurry film on the outer surface of the partially-sintered element body. Satisfying the above-described relationship between the grain diameter of the large grains and the film thickness leads to formation of an excellent slurry film having a sufficient surface roughness.
More specifically, due to size difference between the large grains and the smaller grains, the large grains protrude from the surface level of the slurry film as later explained with reference to FIG. 1A. In this case, the surface roughness of the slurry film can be expressed by a difference between the grain size of large grains and the slurry film thickness. The small grains are completely embedded in the slurry film.
If the slurry film thickness xe2x80x98txe2x80x99 is less than 0.25d, the large grains may fall off the slurry film. On the other hand, if the slurry film thickness xe2x80x98txe2x80x99 is larger than 0.75d, it will be difficult to assure a sufficient bonding force for firmly fixing the outside electrode and the protective layer on the solid electrolytic body.
Furthermore, it is preferable that the manufacturing method of the present invention further comprises the following steps for dipping the outer surface of the partially-sintered element body into the slurry containing the surface-roughing powder:
a step of preparing a slurry tank equipped with a stirrer therein;
a step of rotating the stirrer to cause rotational flow of the slurry in the slurry tank;
a step of dipping the partially-sintered element body in the slurry in a condition where the stirrer is rotating or stopped; and
a step of lifting the partially-sintered element body out of the slurry tank.
This is effective to prevent the surface roughing powder from sedimenting in the slurry. The slurry concentration will be kept uniformly. This assures that the surface roughing powder is adequately attached on the outside surface of the partially-sintered element body.
It is preferable that a plurality of partially-sintered element bodies are hung on a common jig so that the plurality of partially-sintered element bodies can be dipped into the slurry at a time.
It is also possible to hang each partially-sintered element body on an independent jig.
Moreover, a stirrer or a rotary vane is preferably used to stir the slurry.
Furthermore, it is preferable that the manufacturing method of the present invention further comprises the following steps for dipping the outer surface of the partially-sintered element body into the slurry containing the surface-roughing powder:
a step of preparing a slurry tank filled with the slurry;
a step of rotating the partially-sintered element body about its center axis and dipping the partially-sintered element body into the slurry when a stirrer is rotated in the slurry tank, or dipping the partially-sintered element body into the slurry without rotating the partially-sintered element body when the stirrer is not rotated in the slurry tank; and
a step of lifting the partially-sintered element body out of the slurry tank.
In this case, the slurry is adequately stirred. The partially-sintered element body is rotated and dipped into the slurry when rotational flow of the slurry is kept. The partially-sintered element body is not rotated and dipped into the slurry when rotational flow of the slurry substantially disappears. In other words, the dipping treatment of this invention is performed in such a manner that no relative rotation is caused between the partially-sintered element body and the slurry tank.
Thus, the dipping treatment of this invention assures uniform coating of the slurry film containing the surface roughing powder on the outside surface of the partially-sintered element body.